rajmegaurav wrote:

Bunuel - How do we know that the reduced figure is a rectangle. It might even be a parallelogram where the opposite sides of the square are increased in opposite direction and the other opposite sides are reduced in the opposite direction.

Hope I am able to explain the situation

hi, because each internal angle of the square is 90, therefore the new figure will also have the same internal angle of 90 degrees each, that is why the new figure is rectangle and not a parallelogram.